Pilot interference cancellation

ABSTRACT

Techniques for generalized pilot interference cancellation in a communications receiver. In an exemplary embodiment, a residual pilot is cancelled from a post-traffic cancellation signal following initial first-pass pilot cancellation. Residual pilot cancellation is achieved by adding the first-pass cancelled pilot as earlier stored in memory back to the post-traffic cancellation signal, and pilot filtering the resulting signal to generate an improved pilot interference estimate. In an alternative exemplary embodiment, an arbitrary number of iterations may be applied to generate the pilot interference estimate by successively storing each generated pilot interference estimate in memory.

RELATED APPLICATIONS

This application is a continuation-in-part of, and claims priority to,U.S. application Ser. No. 11/334,977, entitled “Reverse LinkInterference Cancellation,” filed Jan. 18, 2006, which claims priorityto U.S. Provisional Application Ser. No. 60/713,517, filed Aug. 31,2005, and U.S. Provisional Application Ser. No. 60/713,549, filed Aug.31, 2005, and U.S. Provisional Application Ser. No. 60/710,370, filedAug. 22, 2005, and U.S. Provisional Application Ser. No. 60/710,405,filed Aug. 22, 2005, the contents of which are hereby incorporated byreference in their entirety.

TECHNICAL FIELD

The present invention relates generally to digital communications, andmore specifically, to techniques for interference cancellation forcommunications receivers.

BACKGROUND

Wireless communications systems are widely deployed to provide varioustypes of communication such as voice, packet data, and so on. Thesesystems may be based on code division multiple access (CDMA), timedivision multiple access (TDMA), frequency division multiple access(FDMA), or other multiple access techniques to allow multiple devices toshare a common communications medium. For example, such systems canconform to standards such as Third-Generation Partnership Project 2(3gpp2, or “cdma2000”), Third-Generation Partnership (3gpp, or“W-CDMA”), or Long Term Evolution (“LTE”). In the design of suchcommunications systems, it is desirable to maximize the capacity, or thenumber of users the system can reliably support, given the availableresources. One technique for increasing the capacity of a communicationssystem is to apply interference cancellation at a receiver to maximizethe received signal-to-interference-and-noise ratio (SINR) of each user.For example, in a communications system based on CDMA, a base stationreceiver may receive a mobile station's traffic signal in combinationwith interference from other mobile stations' traffic signals, as wellas from all mobile stations' pilot signals. Conventional interferencecancellation techniques may initially estimate and cancel interferencefrom all users' pilot signals based on the known contents of the pilotsignals, then estimate and cancel interference from other users' trafficsignals as the contents of such traffic signals become known, e.g.,through decoding the traffic signals.

As traffic signals are decoded, and other users' reconstructed trafficsignals are cancelled from a received signal over time, it is expectedthat the pilot estimates may also be improved over their initial values.It would be desirable to take advantage of this to further improve theperformance of communications receivers.

SUMMARY

An aspect of the present disclosure provides a method for processing acomposite receive signal, the composite receive signal comprising afirst channel and a second channel, the method comprising: estimatingthe first channel to generate a first estimate; cancelling the firstestimate from the composite receive signal; decoding the second channelto generate decoded symbols; re-estimating the first channel based atleast in part on the decoded symbols to generate a second estimate; andcancelling a residual estimate from the composite receive signal, theresidual estimate comprising the difference between the first and secondchannel estimates.

Another aspect of the present disclosure provides a method forprocessing a composite receive signal, the composite receive signalcomprising a first channel and a second channel, the method comprising:successfully decoding the second channel to generate decoded symbols;estimating the first channel based on the first channel to generate afirst estimate; cancelling the first estimate from the composite receivesignal prior to successfully decoding the second channel; estimating thefirst channel based at least in part on the decoded symbols afterdecoding the second channel to generate a second estimate; andcancelling the second estimate from the composite receive signal.

Yet another aspect of the present disclosure provides a method forprocessing a composite receive signal, the composite receive signalcomprising a first channel and a second channel, the method comprising:estimating the first channel to generate a first estimate; cancellingthe first estimate from the composite receive signal; decoding thesecond channel to generate decoded symbols; reconstructing the secondchannel based on the decoded symbols to generate a reconstructed secondchannel; cancelling the reconstructed second channel from the compositereceive signal; re-estimating the first channel after the cancelling thereconstructed second channel to generate a second estimate; andcancelling a residual estimate from the composite receive signal, theresidual estimate comprising the difference between the first and secondchannel estimates.

Yet another aspect of the present disclosure provides a method forprocessing a composite receive signal, the composite receive signalcomprising a first channel and a second channel, the method comprising:estimating the first channel to generate a first estimate; cancellingthe first estimate from the composite receive signal; decoding thesecond channel to generate decoded symbols; re-estimating the firstchannel after the cancelling the reconstructed second channel togenerate a second estimate, the re-estimating based at least in part onthe generated decoded symbols; cancelling a residual estimate from thecomposite receive signal, the residual estimate comprising thedifference between the first and second channel estimates;reconstructing the second channel based on the decoded symbols togenerate a reconstructed second channel; and cancelling thereconstructed second channel from the composite receive signal.

Yet another aspect of the present disclosure provides an apparatus forprocessing a composite receive signal, the composite receive signalcomprising a first channel and a second channel, the apparatuscomprising: a decoder configured to early decode the second channel togenerate decoded symbols; a channel estimator configured to estimate thefirst channel based on the first channel to generate a first estimate,the channel estimator further configured to estimate the first channelbased at least in part on successfully decoded symbols of the secondchannel to generate a second estimate; and a canceller configured tocancel the first estimate from the composite receive signal prior tosuccessfully decoding the second channel, and to cancel the secondestimate from the composite receive signal after successfully decodingthe second channel.

Yet another aspect of the present disclosure provides an apparatus forprocessing a composite receive signal, the composite receive signalcomprising a first channel and a second channel, the apparatuscomprising: means for early decoding the second channel to generatedecoded symbols; means for estimating the first channel based onsuccessfully decoded symbols of the second channel to generate a firstchannel estimate; and means for cancelling the first channel estimatefrom the composite receive signal after successfully decoding the secondchannel.

Yet another aspect of the present disclosure provides acomputer-readable storage medium storing instructions for causing acomputer to successfully decode the second channel to generate decodedsymbols; estimate the first channel to generate a first estimate; cancelthe first estimate from the composite receive signal prior tosuccessfully decoding the second channel; estimate the first channelbased at least in part on the decoded symbols after decoding the secondchannel to generate a second estimate; and cancel the second estimatefrom the composite receive signal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a wireless cellular communications system.

FIG. 2 illustrates example channels received at a base station on areverse link for a CDMA communications system.

FIG. 3 illustrates an example of a transmitter structure and/or process,which may be implemented at an access terminal of FIG. 1.

FIG. 4 illustrates an exemplary embodiment of a receiver, which may beimplemented at a base station of FIG. 1.

FIG. 5 illustrates a method to perform first-pass PIC followed by TIC inthe receiver of FIG. 4.

FIG. 5A illustrates an exemplary embodiment of first-pass pilotcancellation for user #n in the first-pass pilot estimation andcancellation blocks shown in FIG. 5.

FIG. 6 illustrates an alternative exemplary embodiment of a receiveraccording to the present disclosure.

FIG. 7 illustrates a method to perform first-pass PIC and residual PICin the receiver of FIG. 6.

FIG. 7A illustrates an exemplary embodiment of operations performed bythe DACE block and residual PIC block in FIG. 7.

FIG. 7B illustrates exemplary operations performed by DACE block in FIG.7A.

FIG. 7C illustrates an alternative exemplary embodiment of operationsperformed by the DACE and residual PIC block in FIG. 7.

FIG. 8 illustrates a method to perform first-pass PIC using pilot(only)-based channel estimates prior to successful traffic decode, andfirst-pass PIC based on DACE after successful traffic decode, in thereceiver of FIG. 6.

FIG. 8A illustrates a timing diagram of PIC as performed according tothe method of FIG. 8 for user #n.

FIG. 9 illustrates an alternative exemplary embodiment of a receiveraccording to the present disclosure.

FIG. 10 illustrates a method to perform first-pass PIC, DACE-basedresidual PIC, and TIC in the receiver of FIG. 9.

FIG. 11 illustrates a method to perform first-pass PIC, TIC, andresidual PIC in the receiver of FIG. 9.

FIG. 11A illustrates an exemplary embodiment of operations performed ata residual PIC block of the method of FIG. 11.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of thepresent invention and is not intended to represent the only exemplaryembodiments in which the present invention can be practiced. The term“exemplary” used throughout this description means “serving as anexample, instance, or illustration,” and should not necessarily beconstrued as preferred or advantageous over other exemplary embodiments.The detailed description includes specific details for the purpose ofproviding a thorough understanding of the exemplary embodiments of theinvention. It will be apparent to those skilled in the art that theexemplary embodiments of the invention may be practiced without thesespecific details. In some instances, well known structures and devicesare shown in block diagram form in order to avoid obscuring the noveltyof the exemplary embodiments presented herein.

In this specification and in the claims, it will be understood that whenan element is referred to as being “connected to” or “coupled to”another element, it can be directly connected or coupled to the otherelement or intervening elements may be present. In contrast, when anelement is referred to as being “directly connected to” or “directlycoupled to” another element, there are no intervening elements present.

Communications systems may use a single carrier frequency or multiplecarrier frequencies. Referring to FIG. 1, in a wireless cellularcommunications system 100, reference numerals 102A to 102G refer tocells, reference numerals 160A to 160G refer to base stations, andreference numerals 106A to 106G refer to access terminals (AT's). Acommunications channel includes a forward link (FL) (also known as adownlink) for transmissions from a base station 160 to an accessterminal (AT) 106 and a reverse link (RL) (also known as an uplink) fortransmissions from an AT 106 to a base station 160. The AT 106 is alsoknown as a remote station, a mobile station or a subscriber station. Theaccess terminal (AT) 106 may be mobile or stationary. Each link mayincorporate a different number of carrier frequencies. Furthermore, anaccess terminal 106 may be any data device that communicates through awireless channel or through a wired channel, for example using fiberoptic or coaxial cables. An access terminal 106 may further be any of anumber of types of devices including but not limited to PC card, compactflash, external or internal modem, or wireless or wireline phone.

Modern communications systems are designed to allow multiple users toaccess a common communications medium. Numerous multiple-accesstechniques are known in the art, such as time division multiple-access(TDMA), frequency division multiple-access (FDMA), space divisionmultiple-access, polarization division multiple-access, code divisionmultiple-access (CDMA), and other similar multiple-access techniques.The multiple-access concept is a channel allocation methodology whichallows multiple user access to a common communications link. The channelallocations can take on various forms depending on the specificmultiple-access technique. By way of example, in FDMA systems, the totalfrequency spectrum is divided into a number of smaller sub-bands andeach user is given its own sub-band to access the communications link.Alternatively, in TDMA systems, each user is given the entire frequencyspectrum during periodically recurring time slots. In CDMA systems, eachuser is given the entire frequency spectrum for all of the time butdistinguishes its transmission through the use of a code.

While certain exemplary embodiments of the present disclosure may bedescribed hereinbelow for operation according to a CDMA system, one ofordinary skill in the art will appreciate that the techniques mayreadily be applied to other digital communications systems, such asthose based on other multiple-access systems. Such alternative exemplaryembodiments are contemplated to be within the scope of the presentdisclosure.

FIG. 2 illustrates example channels received at a base station on areverse link 200 for a CDMA communications system. In FIG. 2, thevertical axis distinguishes channels based on channelization codes(e.g., Walsh and/or PN codes), while the horizontal axis representstime. Note the example channels are shown for illustrative purposesonly, and are not meant to restrict the scope of the present disclosureto any particular configuration of channels shown.

In FIG. 2, signals are shown received from users #1 through #N, with anarbitrary user designated as user #n. In the implementation shown, user#n's signal includes a pilot and traffic signal, which may containtraffic data bits d_(n)(t). The traffic for each user may be divided intime into a plurality of frames. Note in general, when received by areceiver, frame boundaries of a traffic signal for one user may not betime-aligned with the frame boundaries for other users.

Note as used in this specification and in the claims, the term “traffic”is inclusive of any channel whose data content d_(n)(t) is not known apriori by the receiver. Thus the term “traffic” may encompass both dataassociated with voice traffic in cdma2000 systems, as well as dataassociated with “overhead channels” such as ACK messages, power controlmessages, etc.

In FIG. 2, the pilot for user #n is multiplexed onto a separate codefrom the traffic to allow the receiver to separate the pilot from thetraffic. In some implementations, the pilot may be alternatively orfurther multiplexed using other channelization schemes, e.g., pilot andtraffic may be modulated onto separate quadrature-phase (e.g., I and Q)carriers. At the receiver, the composite receive signal containing thesum of the channelized pilot and traffic signals for all users #1through #N may be processed to recover the traffic associated with eachuser.

In one implementation, the receiver may implement an early decodingscheme, e.g., wherein decoding data bits d_(n)(t) of a traffic frame foruser #n is attempted prior to receiving the entire frame. Mechanisms forearly decoding are further described in, e.g., U.S. patent applicationSer. No. 12/252,544, entitled “Rate Determination,” filed on Oct. 16,2008, assigned to the assignee of the present invention, the disclosureof which is hereby incorporated by reference in its entirety.

FIG. 3 illustrates an example of a transmitter structure and/or process,which may be implemented at an access terminal 106 of FIG. 1 for user#n. The functions and components shown in FIG. 3 may be implemented bysoftware, hardware, or a combination of software and hardware. Otherfunctions may be added to FIG. 3 in addition to or instead of thefunctions shown in FIG. 3.

A data source 300 provides data d_(n)(t) to an encoder 302, whichencodes data bits using one or more coding schemes to provide encodedsymbols. Each coding scheme may include one or more types of coding,such as cyclic redundancy check (CRC), convolutional coding, Turbocoding, block coding, other types of coding, or no coding at all. Othercoding schemes may include automatic repeat request (ARQ), hybrid ARQ(H-ARQ), and incremental redundancy repeat techniques. Different typesof data may be coded with different coding schemes. An interleaver 304interleaves the coded data bits to combat fading.

A modulator 306 modulates coded, interleaved data to generate modulateddata. Examples of modulation techniques include binary phase shiftkeying (BPSK) and quadrature phase shift keying (QPSK). The modulator306 may also repeat a sequence of modulated data or a symbol punctureunit may puncture bits of a symbol. The modulator 306 may also spreadthe modulated data with a Walsh cover (i.e., Walsh code) to form astream of chips. The modulator 306 may also use a pseudo random noise(PN) spreader to spread the stream of chips with one or more PN codes(e.g., short code, long code).

A baseband-to-radio-frequency (RF) conversion unit 308 may convertbaseband signals to RF signals for transmission via an antenna 310 overa wireless communication link to one or more base stations 160.

FIG. 4 illustrates an exemplary embodiment 400 of a receiver, which maybe implemented at a base station 160 of FIG. 1. The functions andcomponents shown in FIG. 4 may be implemented by software, hardware, ora combination of software and hardware. Other functions may be added toFIG. 4 in addition to or instead of the functions shown in FIG. 4.Although interference cancellation at a base station 160 is describedbelow, the concepts herein may be readily applied to an access terminal106 or any other component of a communication system.

One or more antennas 401 receive the reverse link modulated signals fromone or more access terminals 106. Multiple antennas may provide spatialdiversity against deleterious path effects such as fading. Each receivedsignal is provided to a respective receiver or RF-to-baseband conversionunit 402, which conditions (e.g., filters, amplifies, downconverts) anddigitizes the received signal to generate digital samples.

A demodulator 404 may demodulate the received signal to providerecovered symbols. For cdma2000, demodulation tries to recover a datatransmission by (1) channelizing the despread samples to isolate thereceived pilot and traffic onto their respective code channels, and (2)coherently demodulating the channelized traffic with a recovered pilotto provide demodulated data. Demodulator 404 may include a receivedsample buffer 412 (also called joint front-end RAM (FERAM) or sampleRAM) to store samples of the composite receive signal for allusers/access terminals, a Rake receiver 414 to despread and processmultiple signal instances corresponding to distinct “multipaths,” and ademodulated symbol buffer 416 (also called back-end RAM, BERAM, ordemodulated symbol RAM). There may be a plurality of demodulated symbolbuffers 416, each corresponding to a particular user/access terminal.

A deinterleaver 406 deinterleaves data from the demodulator 404.

A decoder 408 may decode the demodulated data to recover decoded databits {circumflex over (d)}_(n)(t) transmitted by the access terminal106. The decoded data may be provided to a data sink 410.

In FIG. 4, decoded data bits {circumflex over (d)}_(n)(t) ofsuccessfully decoded user #n are input to an interference reconstructionunit 460, which includes an encoder 462, interleaver 464, modulator 466and filter 468. The encoder 462, interleaver 464, and modulator 466 maybe similar to the encoder 302, interleaver 304, and modulator 306 ofFIG. 3. The filter 468 forms the decoded user's samples at FERAMresolution, e.g., at 2× chip rate. In an exemplary embodiment, the gainof the filter 468 may be weighted with the channel estimate as derivedfrom, e.g., the pilot estimation, data-augmented channel estimation,and/or other channel estimation techniques further describedhereinbelow. The decoded user's contribution to the FERAM is thenremoved or canceled from the FERAM 412 using traffic cancellation block461, in the process known as traffic interference cancellation (TIC).

Further description is given below of the functions of the FERAM 412 andBERAM 416 in the TIC receiver 400.

In an exemplary embodiment, the FERAM 412 and the BERAM 416 may becircular buffers. The FERAM 412 stores received samples (e.g., at 2×chip rate) and is common to all users. The BERAM 416 stores demodulatedsymbols of the received bits as generated by the demodulator's Rakereceivers 414. Each user may have a different BERAM, since thedemodulated symbols are obtained by despreading with the user-specificPN sequence, and combining across Rake fingers. In an exemplaryembodiment, in the first-pass and residual PIC techniques describedhereinbelow, each Rake finger may estimate its own corresponding pilot,and when the estimated pilot is subsequently cancelled from the FERAM inPIC, it may be cancelled using the offset of the corresponding Rakefinger that derived that estimated pilot. Both a TIC and non-TICreceiver may use a BERAM 416. The BERAM 416 in TIC may store demodulatedsymbols of previous frames that are no longer stored in an FERAM 412.

As further shown in FIG. 4, a first-pass pilot estimation/reconstructionblock 470 is provided. At block 470, first-pass pilot interferencecancellation (PIC) may be performed on the samples in the FERAM 412, sothat demodulation and decoding of each user's traffic signal may proceedwithout interference from the pilot signal of that user and other users.

FIG. 5 illustrates a method 500 to perform first-pass PIC followed byTIC in the receiver 400 of FIG. 4.

At block 502, samples are continuously received and stored into theFERAM 412. In an exemplary embodiment, samples may be written into theFERAM 412 in real time, i.e., the 2× chip rate samples may be written inevery ½ chip. The stored samples in the FERAM 412 are denoted as r(t).

At block 504, the receiver performs first-pass pilot estimation for allusers #1 through #N. As the pilot pattern for all users is known at theBTS, an estimate {circumflex over (p)}_(n)(t) of the received pilotsignal for each user #n may be generated by each Rake finger in the Rakereceiver 414. At block 506, the pilot estimates obtained at block 504may be reconstructed and subtracted from the samples stored in the FERAM412.

FIG. 5A illustrates an exemplary embodiment 560 of first-pass pilotcancellation for user #n in the first-pass pilot estimation andcancellation blocks 504 and 506. In an exemplary embodiment, an instanceof the blocks shown in 560 may be provided, e.g., in each Rake fingerdemodulator of Rake receiver 414 in FIG. 4, wherein a separate Rakefinger is assigned to a distinct multipath associated with each user.

In FIG. 5A, r(t), or the signal stored in the FERAM 412, is coupled to apilot n estimation block 570.n. The pilot estimation block 570.ncomputes an estimate {circumflex over (p)}_(n)(t) of the pilot signalp_(n)(t) associated with user #n, based on the known pilot patternassociated with user #n. Cancellation adder 576 subtracts {circumflexover (p)}_(n)(t) from r(t) to generate {circumflex over (r)}₁(t), alsoreferred to herein as a first composite signal. The first compositesignal {circumflex over (r)}₁(t) may be stored back in the FERAM 410 asthe first-pass pilot cancelled version of the received signal.

In the pilot estimation block 570.n, the signal r(t) is first correlatedwith user #n's pilot pattern by multiplying with a multiplier 590, andaccumulating with an add-and-dump block 591. The output of block 591 maybe provided to a filter 592. Note as defined herein, the “correlation”of a first signal with a second signal may cover the operations ofmultiplying the first signal with the complex conjugate of the secondsignal (or vice versa), and accumulating or filtering the result of themultiplying over a period of time.

Filter 592 may implement, for example, any type of filtering operationfor improving the quality of the pilot associated with user #n, whilede-emphasizing noise and other interference contributions. For example,filter 592 may include a finite-impulse response (FIR) filter, aninfinite-impulse response filter (IIR), and/or a filter havingnon-linear and/or time-varying characteristics as derivable by one ofordinary skill in the art. The output of filter 592 may be reconstructedby “spreading,” e.g., multiplying by user #n's pilot pattern, so thatthe output of the pilot estimation block 570.n may subsequently befurther cancelled from the composite signal r(t).

Note the instance of a pilot estimation block shown in FIG. 5A isprovided for illustrative purposes only. One of ordinary skill in theart will appreciate that alternative instances of the pilot estimationblock shown in FIG. 5A are readily derivable by one of ordinary skill inthe art, and their incorporation into a receiver along with the pilotinterference cancellation techniques of the present disclosure iscontemplated to be within the scope of the present disclosure.

Returning to FIG. 5, following first-pass PIC at blocks 504-506, a groupG of undecoded users is chosen at block 508. In an exemplary embodiment,the group G of users may correspond to those users for whom a sufficientamount of data have been received and stored in FERAM 412. In anexemplary embodiment, G may include a single user, or it may includemultiple users.

At block 510, traffic channel demodulation for users in G is performed,and decoding of traffic is attempted based on samples already received.For example, demodulator 404 demodulates samples of the chosen usergroup's frames for some or all time segments stored in the FERAM 412,according to the users' spreading and scrambling sequence, as well astheir constellation size. Furthermore, the decoder 408 attempts todecode the users' traffic using the demodulated FERAM samples and thepreviously demodulated symbols stored in BERAM 416. Note when G includesmultiple users, decoding of each user in G may be done in parallel withother users, or in succession.

At block 512 of FIG. 5, TIC is performed by reconstructing successfullydecoded traffic data, e.g., by using traffic reconstruction block 460,and subtracting the reconstructed traffic from the FERAM 412. In anexemplary embodiment, decoding success may be determined by, e.g.,checking whether a cyclic redundancy code (CRC) passes an error check.

At block 514, it is checked whether there are more users to decode. Ifyes, the method returns to block 508, and selects a new group G of usersto decode. If no, the method returns to block 504, wherein first-passPIC may be performed for newly received (i.e., prior to first-pass PIC)samples in the FERAM 412.

In an exemplary embodiment, e.g., wherein the group G includes multipleusers, all such users in the group may be decoded together, and thentheir interference contributions subtracted all at once. In analternative exemplary embodiment, e.g., wherein the group G firstincludes one user, and then is updated to include a next user at block514, each user may be decoded and its traffic interference cancelled ina sequential fashion from one user to the next user through the loopbackfrom block 514 to block 508, in a process known as successiveinterference cancellation (SIC). In this embodiment, users later in thedecoding order benefit from the cancellations of users earlier in thedecoding order of the same group.

Whenever a user or group of users is correctly decoded at block 510, itstraffic interference contribution may be subtracted from the FERAM 412,thus increasing the quality of (i.e., reducing the total interferencepresent in) the samples in the FERAM 412. Furthermore, knowledge of thedecoded data associated with the user or group of users may aid inimproved estimation of the channel response, which may lead to moreaccurate PIC by the receiver. Exemplary embodiments implementing thesefeatures are further described hereinbelow.

FIG. 6 illustrates an alternative exemplary embodiment 600 of a receiveraccording to the present disclosure. Similarly labeled elements in FIGS.6 and 4 correspond to blocks having similar functionality, unlessotherwise noted. In FIG. 6, a first-pass and residual pilotestimation/reconstruction block 620 is provided in place of thefirst-pass pilot estimation/reconstruction block 520 in FIG. 5.First-pass and residual estimation/reconstruction block 620 is coupledto a pilot memory 630 for storing pilot interference samples cancelledfrom the FERAM 412, for later use in residual pilot processing asfurther described hereinbelow. The operation of the receiver 600 mayproceed as described in FIG. 7.

FIG. 7 illustrates a method 700 to perform first-pass PIC and residualPIC in the receiver 600 of FIG. 6.

At block 702, samples are continuously received and stored into theFERAM 412.

At block 704, the receiver performs first-pass pilot estimation for allusers. In an exemplary embodiment, first-pass pilot estimation mayproceed as described with reference to, e.g., block 504 in FIG. 5.

Following block 704, at block 705, the pilot estimates {circumflex over(p)}₁(t) through {circumflex over (p)}_(N)(t) are stored in a pilotmemory, e.g., pilot memory 630 in FIG. 6, for later use in residual PIC.Pilot estimates stored in memory for user #1 through #N are also denoted{tilde over (p)}₁(t) through {tilde over (p)}_(N)(t).

At block 706, first-pass PIC is performed by subtracting the pilotestimates obtained at block 704 from the samples r(t) stored in theFERAM 412. At block 708, a group G of undecoded users is chosen.

At block 710, traffic channel demodulation is performed, and decoding oftraffic for users in G is attempted based on samples already received.

At block 712, data-augmented channel estimation (DACE)-based residualPIC is performed for successfully decoded users. In DACE, the pilotpattern is augmented with the successfully decoded traffic to obtain abetter estimate of the channel than possible with only the pilotpattern. DACE is achieved via coherent combining of the pilot patternand decoded data, and may be advantageously used to increase the qualityof PIC, as further described hereinbelow. Residual PIC is performed forusers that successfully decoded, using the channel estimates derivedfrom DACE. In an exemplary embodiment, residual PIC may be configured toaccount for the pilot estimate {tilde over (p)}_(n)(t) earlier stored inthe pilot memory 630 at block 705, which has already been cancelled fromthe samples of the FERAM 412.

At block 714, it is checked whether there are more users to decode. Ifyes, the method returns to block 708, and selects a new group G of usersto decode. If no, the method returns to block 704, wherein first-passPIC may be performed for newly received samples in the FERAM 412.

FIG. 7A illustrates an exemplary embodiment 760 of operations performedby DACE-based residual PIC block 712. An instance of the blocks shown in760 may be provided, e.g., in each Rake finger demodulator of Rakereceiver 414 in FIG. 6, wherein a separate Rake finger is assigned to adistinct multipath associated with each user #n.

In FIG. 7A, {circumflex over (r)}₁(t), or the signal stored in the FERAM412 after first-pass PIC, is coupled to a channel n estimation block770.n. In the channel estimation block 770.n, an adder 771.n first addsback to {circumflex over (r)}₁(t) the pilot signal {tilde over(p)}_(n)(t) for user #n, e.g., as previously stored in the pilot memory630 at step 705. DACE block 772.n then computes an estimate {circumflexover (p)}_(n)″(t) of the pilot signal p_(n)(t) associated with user #n,based on the known pilot pattern, as well as successfully decodedtraffic associated with user #n. The stored cancelled pilot signal{tilde over (p)}_(n)(t) is then subtracted from the output of block772.n using cancellation adder 774.n, to derive a residual differencebetween the already cancelled pilot estimate {tilde over (p)}_(n)(t) andthe DACE-based pilot estimate {circumflex over (p)}_(n)″(t). The outputof 774.n is subtracted from the signal {circumflex over (r)}₁(t) usingcancellation adder 776 to generate a second composite signal {circumflexover (r)}₂(t). The second composite signal {circumflex over (r)}₂(t) maybe written back to the FERAM 412 in place of the signal {circumflex over(r)}₁(t).

FIG. 7B illustrates exemplary operations performed by DACE block 772.n.In FIG. 7B, the incoming signal is correlated with the known pilotpattern, as well as the successfully decoded traffic, for user n, usingmultiplier 790 and add-and-dump block 791. One of ordinary skill in theart will appreciate that by correlating the incoming signal with bothpilot and successfully decoded traffic data, a better estimate of thechannel associated with user n may be obtained than possible with thepilot alone.

One of ordinary skill in the art will further appreciate that inalternative exemplary embodiments (not shown), the correlation performedby blocks 790 and 791 may be implemented using separatemultiply-and-accumulate blocks, e.g., one multiply-and-accumulate forthe complex conjugate of the pilot pattern and onemultiply-and-accumulate for the complex conjugate of the successfullydecoded traffic, and the results added together. In yet anotherexemplary embodiment, the incoming signal may instead be correlated withsuccessfully decoded traffic data alone (without pilot), in a processknown as data-based channel estimation (DBCE). Such alternativeexemplary embodiments are contemplated to be within the scope of thepresent disclosure.

In FIG. 7B, the output of block 791 is provided to a filter 792. Theoutput of filter 792 is used to resynthesize the pilot signal forcancellation from {circumflex over (r)}₁(t) by multiplying with thepilot pattern for user n using multiplier 793.

In the method 700 of FIG. 7, as the DACE-based PIC at block 712 isperformed on receive samples on which first-pass PIC has already beenperformed (at block 704), the PIC at block 712 is also termed “backward”or “residual” DACE-based PIC.

FIG. 7C illustrates an alternative exemplary embodiment 761 ofoperations performed by the DACE 712 and residual PIC block 713. In FIG.7C, an SNR weighting block 778 is provided to weight the pilot estimate(output of DACE block 770.n) prior to cancellation from the FERAM 412 bycancellation adder 776. In the exemplary embodiment shown, the pilotestimate may be scaled as a function of the SNR of the pilot estimate soas to minimize the residual cancellation error. Ideally cancellationonly removes the desired signal; however, practical implementations mayinclude estimation error which effectively adds pilot noise due toestimation errors. Scaling based on SNR can be used to minimize the meansquare power of the residual cancelled pilot, trading off thecancellation of the original pilot versus addition of pilot noise due toestimation errors. One of ordinary skill in the art will appreciate thatsuch weighting techniques may be applied to any PIC schemes describedherein, and that such alternative exemplary embodiments are contemplatedto be within the scope of the present disclosure.

FIG. 8 illustrates a method 800 to perform first-pass PIC using pilot(only)-based channel estimates prior to successful traffic decode, andfirst-pass PIC based on DACE after successful traffic decode, in thereceiver 600 of FIG. 6.

At block 802, samples are continuously received and stored into theFERAM 412.

At block 804, prior to successful traffic decode for a user, the channelfor that user is estimated based solely on the known pilot pattern. Inan exemplary embodiment, such pilot estimation may proceed as describedwith reference to, e.g., block 504 in FIG. 5.

At block 806, first-pass PIC is performed using the pilot-based channelestimates obtained at block 804.

At blocks 808-810, a group of users G is selected and decoded.

At block 812, data-augmented channel estimation (DACE) is performed, andthe resulting channel estimates are used to perform first-pass PIC forthe remainder of the frame for users having traffic successfully decodedat block 810. In an exemplary embodiment, such first-pass DACE-based PICis performed on samples r(t) in the FERAM 412 received after thesuccessful traffic decode, e.g., samples r(t) on which first-passpilot-based PIC has not yet been performed. One of ordinary skill in theart will appreciate that performing DACE for on samples received after asuccessful traffic decode may include, e.g., re-encoding thesuccessfully decoded symbols to generate an expected transmit patternfor the traffic signal to be received for the rest of the frame, andestimating the pilot signal for the rest of the frame by comparing theexpected transmit pattern of the traffic signal, and/or the pilotsignal, with the composite receive signal.

At block 814, it is checked whether there are more users to decode. Ifyes, the method returns to block 808, and selects a new group G of usersto decode. If no, the method returns to block 804.

FIG. 8A illustrates a timing diagram 800A of PIC as performed accordingto the method 800 for user #n. In FIG. 8A, the horizontal axis denotestime, as shown, while the vertical axis denotes an idealized measure ofthe corresponding signal's power. Note the timings and signals shown arefor illustrative purposes only, and are not meant to restrict the scopeof the present disclosure in any way.

In FIG. 8A, an RX signal composed of RX pilot 810A and RX traffic 820Acorresponding to user #n is received at a receiver. RX traffic isformatted in frames, with frame boundaries as marked. Estimated pilot830A corresponds to the receiver's estimate of the RX pilot 810A presentin the RX signal.

The first portion 830A.1 of estimated pilot 830A corresponds to thereceiver's estimate of the RX pilot 810A using pilot-based estimation,as resulting, e.g., from operations done at block 804 of FIG. 8. Thesecond portion 830A.2 of estimated pilot 830A, commencing aftersuccessful traffic decode at 825A, corresponds to the receiver'sestimate of the RX pilot 810A using DACE, as resulting, e.g., from anestimation operation performed at block 812 of FIG. 8.

Post-PIC RX pilot 840A corresponds to the result of cancelling theestimated pilot 830A from the RX pilot 810A. The first portion 840A.1 ofthe post-PIC RX pilot 840A corresponds to cancellation using thepilot-based channel estimate, as resulting, e.g., from operations doneat block 806 of FIG. 8. The second portion 840A.2 of the post-PIC RXpilot 840A corresponds to the cancellation using DACE, as resulting,e.g., from a cancellation operation performed at block 812 of FIG. 8.

In the method 800 of FIG. 8, as the DACE-based PIC at block 812 isperformed on samples received after a successful traffic decode, the PICat block 813 is also termed “forward” DACE-based PIC.

FIG. 9 illustrates an alternative exemplary embodiment 900 of a receiveraccording to the present disclosure. The receiver 900 combines thefirst-pass and residual pilot estimation/reconstruction block 620 withthe traffic cancellation block 461 to further improve the performance ofPIC. In exemplary embodiments, the operation of the receiver 900 mayproceed as described in FIG. 10 or FIG. 11.

FIG. 10 illustrates a method 1000 to perform first-pass PIC, DACE-basedresidual PIC, and TIC in the receiver 900 of FIG. 9.

At block 1002, samples are continuously received and stored into theFERAM 412.

At block 1004, first-pass pilot estimation is performed for all users.In an exemplary embodiment, the operations performed may be similar tothose performed at block 504 in FIG. 5. Pilot estimates {circumflex over(p)}₁(t) through {circumflex over (p)}_(N)(t) obtained at this block arestored in a pilot memory 630 at block 1005, and the pilot estimatesstored in memory are denoted {tilde over (p)}₁(t) through {tilde over(p)}_(N)(t).

At block 1006, first-pass PIC is performed on all users using the pilotestimates obtained at block 1004.

At blocks 1008-1010, a group of users G is selected and decoded.

At block 1012, DACE-based residual pilot estimation is performed on thesamples in the FERAM 412. In an exemplary embodiment, the operationsperformed at block 1012 may be similar to those performed by block 760in FIG. 7A, wherein channel estimation is performed using both the pilotand successfully decoded traffic (DACE). In an alternative exemplaryembodiment, channel estimation may be performed using only thesuccessfully decoded traffic (DBCE). The difference between the DACE (orDBCE)-based pilot estimates and the first-pass pilot estimates may becancelled from the samples r(t) in the FERAM 412.

At block 1013, TIC is performed for successfully decoded users byreconstructing the traffic signals based on the decoded data, andcancelling the reconstructed signals from the samples r(t) in the FERAM412.

At block 1014, it is checked whether there are more users to decode. Ifyes, the method returns to block 1008, and selects a new group G ofusers to decode. If no, the method returns to block 1004.

FIG. 11 illustrates a method 1100 to perform first-pass PIC, TIC, andresidual PIC in the receiver 900 of FIG. 9.

At block 1102, samples are continuously received and stored into theFERAM 412.

At block 1104, first-pass pilot estimation is performed for all users.

At block 1106, first-pass PIC is performed using the pilot estimatesobtained at block 1104.

At blocks 1108-1110, a group of users G is selected and decoded.

At block 1112, TIC is performed for successfully decoded users byreconstructing the traffic signals based on the decoded data, andcancelling the reconstructed signals from the samples r(t) in the FERAM412.

At block 1113, residual PIC is performed for all users on the samples inthe FERAM 412. The residual pilot estimates performed at this blockbenefit from the lesser degree of interference present in the FERAM 412samples r(t) due to TIC performed at block 1112 in the currentiteration, and also to TIC performed at block 1112 in previousiterations of block 1108-1114. In an exemplary embodiment, theoperations performed at block 1113 may be based on a pilot only, and mayutilize a residual PIC block 1160 shown in FIG. 11A, whose operationwill be clear to one of ordinary skill in the art in light of thetechniques previously disclosed hereinabove.

At block 1114, it is checked whether there are more users to decode. Ifyes, the method returns to block 1108, and selects a new group G ofusers to decode. If no, the method returns to block 11104.

In certain alternative exemplary embodiments, blocks 1008, 1108, alongwith blocks 1014, 1114, may be configured to perform PIC and TIC for thesame user or same group of users more than once in succession. This isknown as “iterative” PIC and TIC, and may improve decoding performanceas subsequent passes through the decoders may benefit from accumulatedcancellation of other users' interference. In such exemplaryembodiments, the pilot memory 630 may be further configured to store themost-recently cancelled pilot estimates, rather than only the first-passcancelled pilot estimates, so that subsequent passes of residual PICperformed at, e.g., block 712 of method 700, may correctly account forpilot estimates already cancelled from samples in the FERAM 412. Suchalternative exemplary embodiments are contemplated to be within thescope of the present disclosure.

One of ordinary skill in the art will appreciate that various techniquesdescribed hereinabove may be combined to arrive at alternative exemplaryembodiments not explicitly illustrated or described. For example, in analternative exemplary embodiment, the forward PIC techniques describedwith reference to FIGS. 8 and 8A may be combined with residual PICand/or TIC techniques described with reference to other figures. Suchalternative exemplary embodiments are contemplated to be within thescope of the present disclosure.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the exemplary embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the exemplary embodiments of the invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the exemplary embodiments disclosed herein may beimplemented or performed with a general purpose processor, a DigitalSignal Processor (DSP), an Application Specific Integrated Circuit(ASIC), a Field Programmable Gate Array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theexemplary embodiments disclosed herein may be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. A software module may reside in Random AccessMemory (RAM), flash memory, Read Only Memory (ROM), ElectricallyProgrammable ROM (EPROM), Electrically Erasable Programmable ROM(EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any otherform of storage medium known in the art. An exemplary storage medium iscoupled to the processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium may be integral to the processor. The processor andthe storage medium may reside in an ASIC. The ASIC may reside in a userterminal. In the alternative, the processor and the storage medium mayreside as discrete components in a user terminal.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by acomputer. By way of example, and not limitation, such computer-readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and Blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above may also beincluded within the scope of computer-readable media.

The previous description of the disclosed exemplary embodiments isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these exemplary embodimentswill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other exemplary embodimentswithout departing from the spirit or scope of the invention. Thus, thepresent invention is not intended to be limited to the exemplaryembodiments shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

1. A method for processing a composite receive signal, the compositereceive signal comprising a first channel and a second channel, themethod comprising: estimating the first channel to generate a firstestimate; cancelling the first estimate from the composite receivesignal; decoding the second channel to generate decoded symbols;re-estimating the first channel based at least in part on the decodedsymbols to generate a second estimate; and cancelling a residualestimate from the composite receive signal, the residual estimatecomprising the difference between the first and second channelestimates.
 2. The method of claim 1, the first channel comprising apilot, the second channel comprising data, the estimating the firstchannel comprising: correlating the composite signal with a pilotpattern; and spreading the result of the correlating with the pilotpattern.
 3. The method of claim 1, the first channel comprising a pilot,the second channel comprising data, the re-estimating the first channelcomprising: correlating the composite signal with the pilot pattern andthe decoded symbols; and spreading the result of the correlating withthe pilot pattern.
 4. The method of claim 3, the method furthercomprising storing the first estimate in a pilot memory, there-estimating the first channel further comprising: adding the firstestimate to the composite signal prior to the correlating.
 5. The methodof claim 1, further comprising weighting the residual estimate with afunction of the signal-to-noise ratio associated with the secondestimate prior to cancelling the residual estimate from the compositereceive signal.
 6. The method of claim 1, the composite receive signalfurther comprising a third channel, the method further comprising:reconstructing the second channel based on the decoded symbols togenerate a reconstructed second channel; cancelling the reconstructedsecond channel from the composite receive signal; and decoding the thirdchannel to further generate decoded symbols.
 7. The method of claim 6,the first channel comprising a pilot of a first user, the second channelcomprising traffic data of a first user, the third channel comprisingtraffic data of a second user.
 8. The method of claim 6, comprisingre-estimating the first channel prior to cancelling the reconstructedsecond channel from the composite receive signal.
 9. A method forprocessing a composite receive signal, the composite receive signalcomprising a first channel and a second channel, the method comprising:successfully decoding the second channel to generate decoded symbols;estimating the first channel to generate a first estimate; cancellingthe first estimate from the composite receive signal prior tosuccessfully decoding the second channel; estimating the first channelbased at least in part on the decoded symbols after decoding the secondchannel to generate a second estimate; and cancelling the secondestimate from the composite receive signal.
 10. The method of claim 9,the cancelling the second estimate from the composite receive signalcomprising: cancelling the second estimate from a portion of thecomposite receive signal received after the successfully decoding thesecond channel.
 11. The method of claim 9, the estimating the firstchannel to generate the second estimate further comprising: estimatingthe first channel based on symbols of the second channel received afterthe successfully decoding the second channel.
 12. The method of claim 9,the estimating the first channel based at least in part on the decodedsymbols comprising: generating an expected transmit pattern for thesecond channel received after the successfully decoding the secondchannel, the generating the expected transmit pattern comprisingre-encoding the decoded symbols; and estimating the first channel bycomparing the expected transmit pattern for the second channel with thecomposite receive signal.
 13. The method of claim 12, the estimating thefirst channel based at least in part on the decoded symbols furthercomprising comparing the expected transmit pattern for the first channelwith the composite receive signal.
 14. The method of claim 9, the firstchannel comprising a pilot, the second channel comprising data, theestimating the first channel based at least in part on the decodedsymbols comprising: correlating the composite signal with the pilotpattern and the decoded symbols; and spreading the result of thecorrelating with the pilot pattern.
 15. The method of claim 9, thesecond channel formatted as a plurality of frames, the successfullydecoding the second channel comprising successfully decoding a frame ofthe second channel, the estimating the first channel based at least inpart on the decoded symbols comprising: generating an expected transmitpattern for the remainder of the successfully decoded frame; andcomparing the expected transmit pattern with the composite receivesignal.
 16. A method for processing a composite receive signal, thecomposite receive signal comprising a first channel and a secondchannel, the method comprising: estimating the first channel to generatea first estimate; cancelling the first estimate from the compositereceive signal; decoding the second channel to generate decoded symbols;reconstructing the second channel based on the decoded symbols togenerate a reconstructed second channel; cancelling the reconstructedsecond channel from the composite receive signal; re-estimating thefirst channel after the cancelling the reconstructed second channel togenerate a second estimate; and cancelling a residual estimate from thecomposite receive signal, the residual estimate comprising thedifference between the first and second channel estimates.
 17. Themethod of claim 16, the first channel comprising a known pilot pattern,the re-estimating the first channel comprising correlating the compositereceive signal with the pilot pattern.
 18. A method for processing acomposite receive signal, the composite receive signal comprising afirst channel and a second channel, the method comprising: estimatingthe first channel to generate a first estimate; cancelling the firstestimate from the composite receive signal; decoding the second channelto generate decoded symbols; reconstructing the second channel based onthe decoded symbols to generate a reconstructed second channel;cancelling the reconstructed second channel from the composite receivesignal; re-estimating the first channel after the cancelling thereconstructed second channel to generate a second estimate, there-estimating based at least in part on the decoded symbols; andcancelling a residual estimate from the composite receive signal, theresidual estimate comprising the difference between the first and secondchannel estimates.
 19. An apparatus for processing a composite receivesignal, the composite receive signal comprising a first channel and asecond channel, the apparatus comprising: a decoder configured to earlydecode the second channel to generate decoded symbols; a channelestimator configured to estimate the first channel based on the firstchannel to generate a first estimate, the channel estimator furtherconfigured to estimate the first channel based at least in part onsuccessfully decoded symbols of the second channel to generate a secondestimate; and a canceller configured to cancel the first estimate fromthe composite receive signal prior to successfully decoding the secondchannel, and to cancel the second estimate from the composite receivesignal after successfully decoding the second channel.
 20. The apparatusof claim 19, the channel estimator further configured to: generate anexpected transmit pattern for the second channel received aftersuccessfully decoding the second channel by re-encoding the decodedsymbols; and estimate the first channel by comparing the expectedtransmit pattern for the second channel with the composite receivesignal.
 21. An apparatus for processing a composite receive signal, thecomposite receive signal comprising a first channel and a secondchannel, the apparatus comprising: means for early decoding the secondchannel to generate decoded symbols; means for estimating the firstchannel based on successfully decoded symbols of the second channel togenerate a first channel estimate; and means for cancelling the firstchannel estimate from the composite receive signal after successfullydecoding the second channel.
 22. The apparatus of claim 21, the meansfor estimating comprising: means for generating an expected transmitpattern for the second channel received after successfully decoding thesecond channel by re-encoding the decoded symbols; and means forestimating the first channel by comparing the expected transmit patternfor the second channel with the composite receive signal.
 23. Acomputer-readable storage medium storing instructions for causing acomputer to: successfully decode the second channel to generate decodedsymbols; estimate the first channel to generate a first estimate; cancelthe first estimate from the composite receive signal prior tosuccessfully decoding the second channel; estimate the first channelbased at least in part on the decoded symbols after decoding the secondchannel to generate a second estimate; and cancel the second estimatefrom the composite receive signal.
 24. The computer-readable storagemedium of claim 23, the instructions for causing a computer to estimatethe first channel to generate the second estimate comprising instructionfor causing a computer to: generate an expected transmit pattern for thesecond channel received after the successfully decoding the secondchannel by re-encoding the decoded symbols; and estimate the firstchannel by comparing the expected transmit pattern for the secondchannel with the composite receive signal.